The present invention relates to a burst control circuit for use in a time division multiple access (TDMA) communications system and, more particularly, to a burst control circuit for use in a narrow-band TDMA communications system in which it is required for a carrier wave in a burst form (burst signal) to have not only gently sloped leading and trailing edges but also high-speed rise and fall times.
A conventional burst control circuit is disclosed in a co-pending patent application which claims the priority of the Japanese Patent Application No. 1-166152/89 and corresponds to the U.S. patent application Ser. No. 07/544,431, European Patent Application No. 90112128.5, Canadian Patent Application No. 2,019,935-1 and Australian Patent Application No. 57861/90, and which is assigned to the same assignee as the present application. The disclosed burst control circuit includes a power amplifier preceded by an input level control circuit. A detector circuit detects the output level of the power amplifier to produce a detected signal. The burst control circuit also includes a control signal generator which generates a reference signal having gently sloped leading and trailing edges. The detected and reference signals are voltage compared by a comparator to produce an error signal indicating a voltage difference therebetween. Based on the error signal, two driver circuits control the output levels of the input level circuit and the power amplifier, respectively. By the above-mentioned configuration, the power amplifier produces an output burst signal having gently sloped leading and trailing edges, so that it prevents unnecessary expansion of the spectrum of the output burst signal which may otherwise interfere with other burst signals.
Since the conventional burst control circuit mentioned above controls both the levels of the input level control circuit and the power amplifier, its output burst signal can have an ON/OFF ratio high enough to meet a strict standard, such as a standard set by the Group Special Mobile (GSM) pan-European mobile telephone system. The conventional burst control circuit, however, would not produce an output burst signal having rise and fall times quick enough to meet the standard of, for example, about 20 microseconds required by the GSM system. This can be caused by the slow response speed of a control loop including the input level control circuit, power amplifier, detector circuit, comparator and two driver circuits. The response speed of the control loop may be increased by the use of specially fabricated high-gain comparator and driver circuits. But, this is disadvantageous from the economical viewpoint.